In mobile communication systems, coherent demodulation is often used for data communications between transmitting and receiving devices. Such systems, however, are susceptible to the effects of fading in the communication channel, which causes distortion of the amplitude and carrier phase of the transmitted signal. To compensate for these effects, mobile communication systems using coherent demodulation typically use pilot symbols that are embedded within the transmitted signal. Particularly, the transmitting device inserts the pilot symbols into a data stream at regular intervals to provide known amplitude and phase reference points. The intervals at which the pilot symbols are placed are related to the rate of the fading that is desired to be estimated. The receiving device uses the pilot symbols to estimate the effects of the channel on the carrier phase and/or amplitude of the transmitted signal. Based on these estimated effects, the receiving device is then synchronized to the carrier phase of the transmitted signal.
One example of a coherent demodulation scheme is continuous phase modulation (CPM). In contrast to other coherent digital phase-modulation techniques where the carrier phase at the start of every symbol is not dependent on any previously transmitted symbols, the carrier phase in CPM at the start of each symbol is determined by the cumulative total phase of all previously transmitted symbols, which is known as the phase memory.
A typical method for using pilot symbols to estimate channel effects in a CPM system is taught by Ho et al. in U.S. Pat. No. 7,712,877, and is incorporated herein by reference. In particular, Ho et al. describes a method for generating and inserting data-dependent pilot symbols in a CPM system.
However, there are often instances where the location of a phase reference point falls within a portion of the transmitted signal where insertion of a pilot symbol would be undesirable. For example, in time division multiple access (TDMA) systems, signals are generally structured as data bursts, each of which typically comprises an information field and a synchronization field within the center of the information field. In such TDMA systems, inserting data dependent pilot symbols within the synchronization field is undesirable as doing so prevents the synchronization field from being properly decoded. Simply omitting the pilot symbols is similarly undesirable since doing so reduces the ability of a receiving device to estimate channel effects.
Accordingly, there is a need for a system and method for providing a phase reference point by inserting pilot symbols at a location other than at the location of the phase reference point.
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